It's a climate puzzle that has vexed scientists for more than a decade and added fuel to the arguments of those who insist man-made global warming is a myth.

Since just before the start of the 21st century, the Earth's average global surface temperature has failed to rise despite soaring levels of heat-trapping greenhouse gases and years of dire warnings from environmental advocates, the Los Angeles Times reported.

Now, as scientists with the Intergovernmental Panel on Climate Change gather in Sweden this week to approve portions of the IPCC's fifth assessment report, they are finding themselves pressured to explain this glaring discrepancy.

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The panel, a United Nations creation that shared the 2007 Nobel Peace Prize with Al Gore, hopes to brief world leaders on the current state of climate science in a clear, unified voice. However, experts inside and outside the process say members probably will engage in heated debate over the causes and significance of the so-called global warming hiatus.

"It's contentious," said IPCC panelist Shang-Ping Xie, a professor of climate science at the Scripps Institution of Oceanography at UC San Diego. "The stakes have been raised by various people, especially the skeptics."

Though scientists don't have any firm answers, they do have multiple theories. Xie has argued that the hiatus is the result of heat absorption by the Pacific Ocean — a little-understood, naturally occurring process that repeats itself every few decades. Xie and his colleagues presented the idea in a study published last month in the prestigious journal Nature.

The theory, which is gaining adherents, remains unproved by actual observation. Surface temperature records date to the late 1800s, but measurements of deep water temperature began only in the 1960s, so there just isn't enough data to chart the long-term patterns, Xie said.

Scientists have also offered other explanations for the hiatus: lack of sunspot activity, low concentrations of atmospheric water vapor and other marine-related effects. These too remain theories.

For the general public, the existence of the hiatus has been difficult to reconcile with reports of record-breaking summer heat and precedent-setting Arctic ice melts.

At the same time, those who deny the tenets of climate change science — that the burning of fossil fuels adds carbon dioxide and other greenhouse gases to the atmosphere and warms it — have seized on the hiatus, calling it proof that global warming isn't real.